1. Field of Invention
This invention relates to a microwave moisture sensor which operates on the principle that an emitted microwave is attenuated in rotary resonance with water molecules; and more particularly, to such a sensor comprising at least two transmission and reception systems, each comprising a transmission horn antenna and a reception horn antenna, with the antennas which are disposed on one side of a body being measured being spaced a predetermined distance in a direction perpendicular to the plane of the body.
2. Description of the Prior Art
Microwave moisture sensors are known in the art. One example of such a sensor is shown in FIG. 1. The illustrated sensor includes a transmission horn antenna 4 for emitting a microwave supplied from a microwave oscillator 2, through an isolator 1, toward a measurand 3 (that is, an object being measured), such as a sheet of paper, a reception horn antenna 5 disposed in confronting relation to transmission horn antenna 4 with paper sheet 3 disposed therebetween, a transmission horn antenna 6 for re-emitting the microwave, as received by reception horn antenna 5, toward paper sheet 3, and a reception horn 7 disposed in confronting relation to transmission horn 6 with sheet 3 interposed therebetween. Horns 5 and 6 may be interconnected by a coaxial cable (not numbered). The prior sensor also includes a detector (comprising, for example a Schottky diode) 8 which is energized by a local oscillator, not shown, and which supplies a frequency signal having a frequency which is different from that of the signal from microwave oscillator 2. Detector 8 detects the microwave received by reception horn antenna 7. The sensor further comprises a signal processor 9 supplied with a signal from detector 8, and a preset signal indicative of the basis weight of paper sheet 3, and processes these supplied signals based on a predetermined calibration or working curve to generate a moisture percentage signal. Isolator 1, microwave oscillator 2, horn antennas 4,7 and detector 8 are normally housed in a first metal casing. Horn antennas 5,6 are normally housed in a second metal casing.
As illustrated in FIG. 2, the first and second casings 10, and 11, respectively, are disposed in confronting relation on a centrally open frame 12 and jointly constitute a detector head 13. First and second casings 10,11, which may be of metal, synchronously scan or reciprocally travel over an interval or distance between limits L.sub.1 and L.sub.2, for detecting signals. The scanning direction extends substantially transversely of the moving direction of paper sheet 3, which moves in the direction of arrow A.
While detector head 13 scans sheet 3, the transmission and reception horn antennas are kept at a constant distance l, as shown in FIG. 1, from sheet 3, with the transmission and reception horn antennas 4 and 5, 6 and 7 being equidistant from the travel of the paper sheet 3 as positioned in FIG. 1.
Moisture measurement is effected by the microwave moisture sensor while the interval L.sub.1 -L.sub.2 is being scanned by detector head 13. At this time, a microwave emitted from transmission horn antenna 4 is propagated through a path from transmission horn antenna 4, to paper sheet 3, to reception horn antenna 5, via coaxial cable not numbered to transmission horn antenna 6, to paper sheet 3, to reception horn antenna 7, and is finally detected by detector 8. Signal processor 9 is fed with a signal, as detected by detector 8 and a preset signal, representative of the basis weight of paper sheet 3, and processes the supplied signals based on a calibration or working curve, thereby to generate a moisture percentage signal.
A voltage V.sub.1 ( of a standing wave) in the propagation path can be derived on the basis of a wave motion from the following equation, provided there is no attenuation in the propagation path. EQU V.sub.1 =V.sup.+ e.sup.-j.beta.x +.GAMMA.V.sup.+ e.sup.j.beta.x ( 1)
wherein V.sup.+ is the output voltage of the microwave oscillator, .GAMMA. is the reflectivity, .beta. is the phase constant, and x is the propagation path length from the microwave oscillator. The amplitude signal .vertline.V.sub.1 .vertline. in equation (1) is expressed by following equation (2), while the ratio (standing wave ratio) of the maximum value .vertline.V.sub.1max .vertline. to the minimum value .vertline.V.sub.1min .vertline. of amplitude signal .vertline.V.sub.1 .vertline. is expressed by the following equation (3). The phase, amplitude and other parameters of tne voltage V.sub.1 vary with vibrations of paper sheet 3. ##EQU1##
The conventional microwave moisture sensor is, however, disadvantageous, in that the measurement errors are increased since the detector also detects influences due to vibrations of paper sheet 3 which occur during its movement.
The actual propagation path of the microwave includes, in addition to the illustrated propagation path, paths in which the microwave is reflected between confronting surfaces of casings 10,11, and between these confronting surfaces and sheet 3. Since such additional propagation paths vary due to vibrations of the travelling paper sheet 3, to thereby change the amount of microwaves falling on tne respective reception horn antennas, measurement errors are also increased.